The use of diamond abrasives in metal bonds is well known. U.S. Pat. No. Re. 21,165; U.S. Pat. Nos. 2,737,454; and 2,828,197 disclose abrasive bodies comprising diamond in metal bonds of various copper alloys and bronze bonds.
Metal bonded agglomerated or aggregated abrasives are also well known. U.S. Pat. No. 2,216,728 discloses various forms of metal and glass bonded aggregates. U.S. Pat. No. 3,955,324 teaches the use of a metal/diamond aggregate in which diamond is completely surrounded by metal. U.S. Pat. No. 4,024,675 describes sintered diamond/metal aggregates containing an additive (titanium, zirconium, chromium, vanadium, or silicon) in an amount of 5 to 10 percent of the metal powder. The additive is said to impart the required degree of wettability of the diamond grains with the metal and also cohesion of individual grits to each other.
U.S. Pat. No. 4,246,006 (incorporated herein by reference) discloses copper/silver alloy metal binders used in metal/diamond aggregates which aggregates include a wetting binder as in the '675 patent. The metal alloy employed may be in powdered form for the starting mixture; however, it is preferred that the individual alloy components are used in the starting mixture in order to produce the alloy in situ. The amount of metal alloy powder used in the starting mixture for the aggregate is generally between 40 and 60 weight percent of the total, and the wetting agent is typically between 5 and 15 weight percent of the metal. South African Patent Application No. 77/7153 is similar to U.S. Pat. No. 4,246,006 except sintering is done in a collapsible container.
The diamond used in the aggregates of the '006 patent is selected from one or more of three specific types defined in the patent in terms of Friatest Index, metal content, color, shape, and surface roughness. The Friatest is one of a variety of measures of the strength or friability (i.e. susceptibility to fracture) of the diamond. Typically in such friability tests, a sample of the diamond particles of a particular size is placed in a capsule with a hard steel ball and shaken for a pre-determined period of time. The abrasive particles are then removed from the capsule and screened through the next smaller screen size in relation to the smaller of the two screens used for determining the screen size of the original particles. That is, if 60/80 mesh (250/180 micron) diamond were being tested, the screen used would be the next standard size smaller than 80 mesh or 180 micron. The amount of diamond retained on the screen divided by the weight of the original sample yields a value (toughness index) which indicates the proportion of the diamond which was not broken down to smaller size.
The '006 patent states that it is preferable that the diamond particles used are all fine and have a size smaller than about 125 microns. A powdered mixture of diamond particles and metal is sintered under non-oxidizing conditions, above the liquidus temperature of the metal alloy (typically at a temperature in the range of 700.degree. to 1200.degree. C.) for a time between 10 and 20 minutes. The sintered mass is then cooled to produce an ingot which is crushed and classified to produce the aggregate grits of desired size. The crushing is preferably by a means which reduces the size of the ingot primarily in the shear mode rather than by compression. This is also preferred for the invention described herein. Suitable crushers would be a jaw crusher or a laboratory mill such as the Alpine perplax mill by Alpine American Corp., Natick, Mass.
There is a currently available commercial aggregate diamond grit, CDA-M from DeBeers Industrial Diamond Division of Johannesburg, South Africa. CDA-M particles are relatively coarse, having a size which centers around 80 mesh (180 microns). The much smaller constituent diamond grits are said to cover a broader range of sizes, approximately 140 U.S. mesh (106 microns) and finer. The metal bond alloy used in CDA-M grit is said to account for about 55 weight percent of the whole particle.
The abrasives art recognizes various classes of industrial diamond, the three major classes being resin diamond, metal bond diamond, and saw diamond. Resin bond diamond is suitable for resin bond or vitreous bond grinding wheels and comprises friable, irregular crystals usually coated with nickel alloy or copper. Metal bond diamond is used in metal matrix bonds or plated tools and comprises medium-toughness, regular crystals with a color range from yello-green to light yellow. Crystal inclusions are generally low, but some heavily included crystals can be present. Saw diamond, used in sawing and drilling stone, concrete and refractories, comprises tough (low friability), blocky, cubooctahedral crystals with predominantly smooth faces. The saw diamond crystal is transparent or translucent having a color ranging from light yellow to medium yellow-green. Due to the higher impact strength or toughness of saw diamond, its fracture during sawing operations is minimized and wear occurs largely by abrasion processes.
Saw diamond is generally more blocky in shape (i.e. has a lower aspect ratio) than metal bond or resin bond diamond. Diamond may be classified by shape separation on a shape sorting machine which separates the diamond crystals according to their aspect ratio. Such machines comprise a vibrating inclined table which causes the particle to segregate according to shape, and it collects various fraction of the particles in separate cups or bins. Such machines are well known to the art, and a description may be found in Dyer, H. B., "EMB Natural Diamonds," Industrial Diamond Review, (August, 1964) page 192-196.
Metal bonded agglomerated or aggregated diamond abrasives can exhibit higher grinding ratios under certain conditions than nickel or copper coated resin bond type abrasives. It was in an effort to improve upon the aggregates of U.S. Pat. Nos. 4,024,675 and 4,246,006 that this invention was achieved.